Modern computing systems utilize virtual (or logical) memory. Virtual memory is a concept that, when implemented by a computer and its operating system, maps virtual storage addresses to physical storage addresses, which generally frees software programmers from being concerned about the availability of data storage. Virtual memory also allows programmers to use a contiguous range of memory, or storage addresses for stored data, that may be larger than the amount of physical memory (RAM) available to the computer.
In addition to managing the mapping of virtual storage addresses to real storage addresses, a computer implementing virtual memory also manages storage swapping between active storage (RAM) and hard disks or other high volume storage devices. Virtual storage is typically committed in units called “pages” that form virtual memory regions ranging from a few kilobytes up to several megabytes in size. Managing storage in this manner allows multiple programs to safely run simultaneously on the computer.
In certain programming languages, including C and Pascal, heaps are used to manage allocation of virtual memory. A heap is an area of pre-reserved storage that a program process can use to store data in some calculatable amount that won't be known until the program is running. For example, a program may accept different amounts of input from one or more users for processing and then perform processing on all the input data at once. Having a certain amount of heap storage already obtained from the operating system facilitates storage management and is generally more efficient than asking the operating system for storage every time it is needed. A process manages its allocated heap by requesting heap memory blocks as needed and returning the blocks when they are no longer needed. Heaps often become fragmented, wasting memory in the virtual address space and, correspondingly, wasting space in physical memory. Heap fragmentation can also contribute to memory overhead that can result in “thrashing” and poor system wide performance in a heavily-loaded system.
To aid application developers in optimizing memory usage, it would be beneficial to provide a notification to the developers when memory is not being efficiently used.